A new research program is proposed dealing with the metabolism of cholesterol and vitamin A (retinol). The proposed research is designed to explore the esterification reactions for cholesterol and retinol in microsomal membranes and in cultured cells. Proposed experiments address the nature of the retinol esterification reaction in two tissues which are active in lipid metabolism and lipid secretion: liver and the lactating mammary gland. Retinol esterification in these tissues will be compared. Using a newly developed assay based on the formation of 3H-retinyl esters from 3H-retinol, experiments will be conducted to describe the reaction and its dependence on specific cofactors. Experiments will also be conducted to determine the source of the fatty acid used in retinol esterification. These experiments will make use of lipid separations by high-pressure liquid chromatography and gas-liquid chromatography. The assays that will be developed for retinyl ester and cholesteryl ester synthesis will be applied in a set of experiments designed to compare these enzymic processes in in liver microsomes. Other studies seek to explore the distribution of cholesterol and retinol esterifying enzymes in membranous fractions of milk, and within the microsomal membrane itself. Experiments are also proposed to compare the metabolism of cholesterol and retinol in isolated cultured hepatocytes and hepatoma cells. The primary objective of these studies will be to determine whether the uptake, esterification and intracellular storage of retinol are regulated in ways similar to those of cholesterol in these cells.